An air cleaner for the contemplated use conventionally comprises a housing having air inlet and outlet ports, an outer, principal filter cartridge and an inner, safety filter cartridge. Typically, the outlet port is central or axial, and air flows inward radially through the cartridges. The latter are usually mounted in the housing by means of a central support yoke, being clamped between an end of the housing and a closure member centrally traversed by an extension of the yoke.
This is a sturdy and practical arrangement, and one which is quite satisfactory when the filter cartridges are substantial enough to tolerate the necessary axial compressive forces without deterioration. For this purpose, the filter cartridges, in which the medium itself may be pleated paper, are so constructed that the paper is contained between inner and outer cylindrical supporting and protective shells of perforated metal, extending between end caps like the paper medium itself.
It is becoming increasingly desirable, however, to make at least the principal filter cartridge without the perforated metal shells. These cartridges are simply replaced and discarded when they become filled with dirt, and the cost of this procedure is considerably increased if metal shells must be discarded with the paper medium. Since the safety filters seldom need replacement, the expense of providing them with inner and outer shells is not intolerable.